The proportion of children and adolescents in the population
declined, while the proportion of older people increased
throughout the century.

These two phenomena follow mechanically from the falling birth rate and rising
average length of life. As the birth rate falls, the ratio of children to adults
necessarily
diminishes and the average age of the population rises. As people live
longer on average, the proportion of the population at older ages necessarily
becomes larger.

Because the decline in the birth rate was almost continuous (with the exception
of the baby boom) and the lengthening of lifetimes fully continuous, the
proportion
of children and adolescents in the population decreased steadily from 44
percent in 1900 to 29 percent in 1998. If the birth rate declines further or
remains stable and average lifetimes continue to lengthen, the youthful
component
of the population will continue to decrease. The Census Bureau’s middle
series projection indicates that children and adolescents will constitute barely a
fifth of the population by 2020.

These changes at both ends of the age spectrum did not have much impact on the
relative size of the intermediate group between the ages of twenty and fifty-nine.
This segment represented roughly 50 percent of the population throughout the
twentieth century, and this is not expected to change much in the twenty-first.
That percentage is important because it represents a ratio of 1:1 between people
of working age, the great majority of whom are economically active, and their
individual or collective dependents.

HS series A 119–134. See also CB, Population Estimates Program, Population
Division, “Resident Population Estimates of the United States by Age and Sex” at
www.census.gov/population/estimates/nation/intfile2-1.txt (accessed August 20,
2000).